In OFDM communications systems the frequencies and modulation of a frequency-division multiplexing (FDM) communications signal are arranged orthogonal with each other to eliminate interference between signals on each frequency. In this system, low-rate modulations with relatively long symbols compared to the channel time characteristics are less sensitive to multipath propagation issues. OFDM thus transmits a number of low symbol-rate data streams on separate narrow frequency subbands using multiple frequencies simultaneously instead of transmitting a single, high symbol-rate stream on one wide frequency band on a single frequency. These multiple subbands have the advantage that the channel propagation effects are generally more constant over a given subband than over the entire channel as a whole. A classical In-phase/Quadrature (I/Q) modulation can be transmitted over individual subbands. Also, OFDM is typically used in conjunction with a Forward Error Correction scheme, which in this instance, is sometimes termed Coded Orthogonal FDM or COFDM.
An OFDM signal can be considered the sum of a number of orthogonal subcarrier signals, with baseband data on each individual subcarrier independently modulated, for example, by Quadrature Amplitude Modulation (QAM) or Phase-Shift Keying (PSK). This baseband signal can also modulate a main RF carrier.
OFDM communications systems have high spectrum efficiency (a high number of bits per second per Hz of bandwidth), simple mitigation of multi-path interference, and an ease in filtering noise. OFDM communications systems suffer, however, from time-variations in the channel, especially those which cause carrier frequency offsets. Because the OFDM signal is the sum of a large number of subcarrier signals, it can have a high peak-to-average amplitude or power ratio. It is also necessary to minimize intermodulation between subcarrier signals, which can create self-interference in-band, and create adjacent channel interference. Carrier phase noise, Doppler frequency shifts, and clock jitter can create Inter-Carrier Interference (ICI) for closely frequency-spaced subcarriers. The subcarriers are typically transmitted at assigned frequency locations within a transmission spectrum. Over the duration of the transmission of an OFDM signal, the average power per subcarrier is significant, and can be easily detected and intercepted, which is undesirable to a system requiring Low Probability of Detection (LPD) and Low Probability of Interception (LPI) characteristics. The receiver that is to receive the OFDM signal requires a minimum signal-to-noise ratio (SNR) per subcarrier in order to demodulate and decode the signal with an acceptably low bit error rate (BER). If there is other unwanted energy within the transmission spectrum, the SNR can decrease causing an increase in BER. Said unwanted energy can be unintentional noise from other sources. In this case the noise is referred to as “interference” and the sources are referred to as “interferers.” If the unwanted energy corrupting the transmission is transmitted intentionally by some third party source known as a jammer, it is referred to as a jamming signal. The conventional OFDM signal is susceptible to such interferers and jammers because of the required minimum SNR per subcarrier for an acceptably low BER. Further, frequency selective fading in the channel causes transmission nulls within the OFDM signal's transmission spectrum, which selectively reduce the SNR on certain subcarriers within those nulls, depending on their frequency location, leading to an undesirable increase in BER.